In the majority of countries, legislation is now in force making it an offense to be in control of a motor vehicle when the body alcohol concentration is above a certain level. This is sometimes expressed in terms of the blood/alcohol concentration or the breath/alcohol concentration. Charts have been produced intended to give drinking drivers some guide as to what alcohol consumption will keep them within the law. However, they are unable to take account of the many variables which affect the alcohol concentration in a person's body. Coin freed breath-testing machines are available in some pubs, but they are not well used and are susceptible to "mouth alcohol" and incorrect usage. Proposals to market individual breath-testers to the general public have failed because of the expense of the units and the difficulty of ensuring that they are used exactly in accordance with the manufacturers' instructions.